introduce scopes identified by types

[RonnyPfannschmidt]

currently we have scopes as opaque strings, that somewhat relate to python things,
but clearly dont match the types of collection nodes we have

we should introduce a more nuanced mechanism so that we can better handle non-python nodes for
both code checkers and different language plugins (bdd style languages and spec type tests in yaml files come to mind)

[ktbarrett]

This feature was exactly what I was thinking for introducing node and invocation scopes as proposed in pytest-dev/pytest-xdist#271.